In the coming year we will pursue the following research goals. We hope to characterize the stoichiometric composition of the histidyl-tRNA ATP phosphoribosyltransferase complex exactly. We hope to determine whether there is something covalently different about enzyme which can bind histidyl-tRNA compared to that which cannot after preparatively separating complexed from uncomplexed enzyme, if possible. We hope to further study ligand control of the histidyl-tRNA enzyme complex to determine what ligands can dissociate the preformed complex. We will reexamine the specificity of complexation under our recently attainable more physiological pH 7.5 conditions. We anticipate increased specificity for histidyl-tRNA compared to unaminoacylated tRNAHis and noncognate tRNA than previously demonstrated at pH 6.5 by Goldberger and coworkers. We hope to determine whether histidyl-tRNA binding to ATP phosphoribosyltransferase inhibits its enzymatic activity, and conversely, whether ATP phosphoribosyltransferase binding to tRNAHis inhibits aminoacylation by histidyl-tRNA synthetase. We will determine the steady-state inhibition patterns of guanosine tetraphosphate competed against each substrate to see whether this nucleotide acts at the active site. We propose to test for dominant regulation of the histidine operon by ATP phosphoribosyltransferase in vivo under growth conditions resulting in coupled histidine and guanosine tetraphosphate levels. BIBLIOGRAPHIC REFERENCES: D. Morton and S. M. Parsons, "Biosynthetic Direction Substrate Kinetics and Product Inhibition Studies on the First Enzyme of Histidine Biosynthesis, Adenosine Triophosphate Phosphoribosyltransferase", Arch. Biochem. Biophys., 175, 677 (1976). D. P. Morton and S. M. Parsons, "Synergistic Inhibition of ATP Phosphoribosyltransferase by Guanosine Tetraphosphate and Histidine", Biochem. Biophys. Res. Comm., 74, 172 (1977).